1. Field of the Invention
The present invention relates generally to a process for the removal of mercaptans from petroleum distillate streams. More particularly the invention relates to a process wherein the petroleum distillate contains diolefins which are selectively reacted with the mercaptans to form sulfides. Most particularly the invention relates to a process wherein the reaction of the mercaptans with the diolefins is carried out simultaneously with a fractional distillation to remove the sulfides, and thus the sulfur, from the distillate.
2. Related Information
Petroleum distillate streams contain a variety of organic chemical components. Generally the streams are defined by their boiling ranges which determine the compositions. The processing of the streams also affects the composition. For instance, products from either catalytic cracking or thermal cracking processes contain high concentrations of olefinic materials as well as saturated (alkanes) materials and polyunsaturated materials (diolefins). Additionally, these components may be any of the various isomers of the compounds.
The petroleum distillates often contain unwanted contaminants such as sulfur and nitrogen compounds. These contaminants often are catalyst poisons or produce undesirable products upon further processing. In particular, the sulfur compounds can be troublesome. The sulfur compounds are known catalyst poisons for naphtha reforming catalysts and hydrogenation catalysts. The sulfur compounds present in a stream are dependent upon the boiling range of the distillate. Mercaptans are most commonly found in the lower boiling range distillates such as the "front end" of a full boiling range naphtha.
The most common method of removal of the sulfur compounds is by hydrodesulfurization (HDS) in which the petroleum distillate is passed over a solid particulate catalyst comprising a hydrogenation metal supported on an alumina base. Additionally copious quantities of hydrogen are included in the feed. The following equations illustrate the reactions in a typical HDS unit:
RSH+H.sub.2.fwdarw.RH+H.sub.2 S (1) EQU RCl+H.sub.2.fwdarw.RH+HCl (2) EQU 2RN+4H.sub.2.fwdarw.RH+NH.sub.3 (3) EQU ROOH+2H.sub.2.fwdarw.RH+H.sub.2 O (4)
Typical operating conditions for the HDS reactions are:
Temperature, .degree. F. 600-780 Pressure, psig 600-3000 H.sub.2 recycle rate, SCF/bbl 1500-3000 Fresh H.sub.2 makeup, SCF/bbl 700-1000
As may be seen the emphasis has been upon hydrogenating the sulfur and other contaminating compounds. The sulfur is then removed in the form of gaseous H.sub.2 S, which in itself is a pollutant and requires further treatment.
The naphtha stream from either a crude distillation column or fluid catalytic cracking unit is generally fractionated several times to obtain useful cuts. The full boiling range naphtha (C.sub.4 -430.degree. F.) may first be debutanized to remove C.sub.4 and lighter materials as overheads in a debutanizer, then depentanized to remove C.sub.5 and lighter materials as overheads in a depantanizer (sometimes referred to as a stabilizer) and finally split into a light naphtha (110-250.degree. F.) and a heavy naphtha (250-430.degree.).
U.S. Pat. No. 5,510,568 (Hearn) discloses a process for removing mercaptans from a distillate feed in a distillation column reactor by reacting the diolefins in the feed to form sulfides in the presence of a Group VIII metal catalyst and hydrogen. U.S. Pat. No. 5,321,163 (Hickey et al) discloses a similar process with an etherification zone also positioned in the distillation column reactor. In both of these processes the distillate feed is fed below the catalyst bed.
One advantage of the present invention is that the present process allows the use of existing debutanizers which are higher pressure than existing gasoline splitters thus providing the appropriate temperatures in the thioetherification bed not obtainable in the low pressure gasoline splitters. The complete gasoline stream through the end point is contacted with the thioetherification catalyst, thus the mercaptans throughout the gasoline range are reacted to heavier thioetherification. Other advantages and features of the present invention will become apparent from the following description.